Why should an ultrarunner care about sprinting? Most people who run ultras found that, the longer and slower a race, the better they did and their idea of a sprint is anything faster than a crawl. Well, there are a couple of reasons:
To run at top speed, one needs good running form and sprinting can help force one out of bad habits picked up with megamileage. Many good ultra runners use the shuffle stride; it's hard to describe but you know it when you see it - some look like they're squatting (it's not jogging, which has a lot of vertical motion and where the toes never go furhter forward than the knees). There are some fast shufflers... Alberto Salazar and Derek Clayton could shuffle 5 minute miles for two hours. The shuffle is very efficient, but it's counterproductive in some race situations and sprinting will reteach one's muscles how to run without shuffling.
Some people have noticeable flaws in their style. Jim Ryun rolled his head in circles, Bill Rodgers loops his left arm in big circles and I don't change the angle of my elbows enough, causing my shoulders to bob back and forth. Coaches will usually try to change these styles, but it may not be necessary: if the flaw doesn't get worse the faster one runs, it isn't a big deal. My form is almost perfect at top speed. Have someone watch you when you run fast and find out for yourself if you need to work on form.
Range of Motion
Long distance usually means inflexibility, especially in the hamstrings and back. I'm a firm believer that, any benefit one can get from cross-training, one can get from some type of running. Sprinting requires full range of motion, which in turn requires flexibility. So, regular sprinting can substitute for stretching (if you're not used to sprinting, make sure you stretch first!).
This isn't very important in ultras, as 4-5 seconds in several hours is nothing. Still...
The physiology of sprinting
There are people with McArdle's Disease who can walk and jog slowly, but if they try to sprint, get terrible cramps after a couple of seconds. Their bodies can store glycogen, but can't use it. So, if they aren't burning fat, sugar, protein or anything else, how can they manage those few seconds? Muscles store a few seconds worth of emergency energy in the form of creatine phosphate. (By the way, the pain isn't from lactic acid, since they don't make it; it's actually from adenosine diphosphate accumulation.)
An experiment: Exhale all the air from your lungs and see how long it takes before you have to inhale. This is about how long you can sprint using this energy (15 seconds for me).
Your body will restore this energy very quickly as soon as you stop moving by transferring energy from carbohydrate metabolism back to creatine. Thus, one can sprint more than once.
Your body makes creatine from arginine in your diet, but increased arginine doesn't lead to increased creatine. Bodybuilders found that one can supplement with creatine and one's muscles will absorb more than usual; because the muscles store water with creatine, this makes the muscles swell (not grow, as the bodybuilders claim).
Can creatine supplementation help a runner? Probably only sprinters would really benefit. I've tried it; didn't do anything for me. It should be noted that the creatine supplements sold keep changing formulation, because it's hard to trick the body into storing what it doesn't need; it's much more unnatural than carbohydrate loading.
Sprinting is actually fun. We tend to forget that, as we usually try to sprint much too far and develop muscle pain because of it. Try throwing in a four or five second burst of speed once a mile some day when you're bored - you might find, as I did, that the overall run becomes much faster than expected and doesn't feel tough at all. Plus, there's a carry-over effect: the next day's run is usually faster too!
Sprint only when warmed up. Do it only on even terrain and, until used to it, on flat ground. Experienced sprinters sprint on slight downhills for added benefit (not steep and NOT on loose rock among tree roots!). They also tend to have sore glutes the next day.
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